Reuters: Diplomats meeting in Vienna this week hope to take a major step toward a deal under the Montreal Protocol to decrease the use of a potent greenhouse gas, in what could be the most significant measure to combat global warming since last year's Paris climate agreement. Officials from nearly 200 countries are trying to hammer out details of an agreement to cut the use of hydrofluorocarbons (HFCs) used in heating and air conditioning by amending the ozone-protection treaty that went into force in 1989....